In chemical and pharmaceutical laboratories, it is often necessary to dispense reagents or products in powder form, in particular to prepare samples of powder combinations for serial tests. With a large number of powdered products to be tested, the successive dispensing and weighing of powder doses represents a painstaking task which requires a considerable amount of time and attention from the staff involved.
In addition to laboratory applications, there are industrial production processes where the same fill quantity of a powdery substance has to be dispensed thousands of times into identical containers.
Thus, there is a strong need for apparatus through which the dispensing of measured doses of powder can be reliably and efficiently automated in laboratories as well as production facilities.
The central element of a metering apparatus of the kind that this invention relates to is a metering device with a holding container for the powder to be dispensed and, attached to the holding container, a metering head with a discharge orifice. When the metering device is in its working position in the metering apparatus, the holding container is on top and the metering head at the bottom, so that the powder runs in a controlled stream from the discharge orifice into a receiving container. The receiving container may be positioned on a weighing device which is part of the metering apparatus and which sends a feedback signal to a shutter device that controls the aperture of the discharge orifice, so as to gradually reduce and shut off the discharge orifice when the powder dispensed into the receiving container reaches its target weight.
Alternatively, instead of setting the receiving container on a weighing device, the state of the art offers other suitable means for sensing the fill level of the receiving container, for example an optical sensor that sends a feedback signal to the powder-metering device when the fill level in the receiving container reaches a target level. In the kind of metering device that is envisaged by the present invention, the size of the powder samples being metered out is typically in the range from 0.5 milligrams to 5 grams. Especially for small samples, the precision required is ±0.1 mg or even less.
Some powders will run easily in an even stream out of a small orifice, comparable to the sand in an hourglass. In this case, the powder flow can indeed be controlled simply by varying the cross section of the orifice with an appropriate shutter device. However, in many powdery materials the particles have a tendency to stick together which is known as cohesion, and the powder may also have a degree of volume compressibility, so that the material can become compacted and, as a result, clog up the discharge orifice or leave the orifice in lumps rather than in an easy-flowing fine stream. As a typical example, corn starch exhibits cohesion as well as compactibility to a very high degree and is therefore often used as a test material to evaluate the capabilities of powder-metering apparatus.
The problems that the cohesion and compactibility—often simply referred to as “stickiness”—of powders causes in powder-metering devices has long been known and numerous solutions have been proposed, including for example: Archimedean feed screws to move the powder material to and expel it from the discharge orifice; stirring and scraping devices to loosen the powder, to scrape it off the walls of the container and metering head and off the feed screw and to collapse bridges and cavities that can form in the powder inside the holding container and metering head; tapping and vibrating devices serving likewise to loosen the powder as well as keep it from adhering to the walls of the container and metering head.
To evaluate and compare the capabilities of state-of-the-art powder-metering devices as well as of the device of the present invention, the applicant has developed a rating scale for the stickiness of powders, wherein a free-flowing powder (as the aforementioned sand in an hourglass) is assigned a numerical grade of 1 and cornstarch is assigned a grade between 7 and 8. When the same cornstarch was used over and over again in the tests conducted by the applicant, the stickiness increased gradually from 7 to 8 as the material absorbed humidity from the air and/or when the powder grains were crushed by the feeding, stirring and scraping elements in the metering devices.
A powder-dispensing device disclosed in Great Britain Pat. B 701,572 has a cylindrical hopper with a conical, funnel-shaped bottom portion. A vertically oriented Archimedean feed screw extends downward through the discharge passage of the funnel. A stirrer/scraper arm moves concentrically, but with the opposite sense of rotation, about the feed screw shaft, thereby stirring the powder in the holding container as well as scraping powder off the feed screw and the funnel wall.
Another powder-dispensing device, which is disclosed in French Pat. 2 607 794, has a funnel-shaped hopper. The discharge orifice at the bottom of the hopper has a plunger valve with a plunger shaft descending vertically from an actuating mechanism above the hopper. A helix-shaped feeder snake is arranged to rotate concentrically about the plunger shaft with a slight clearance from the latter.
In the two foregoing examples and also in general, the use of Archimedean screws for cohesive and compactable powders can be problematic in that the helicoidal grooves of the screw can become firmly clogged with powder so that the Archimedean screw and the powder in its grooves rotate together as a solid cylinder and, as a result, no powder comes out of the discharge orifice.
As a further example of the prior art, a dosage-dispensing device which is disclosed in German laid-open application 198 41 478 A1 has a cylindrical hopper with a funnel-shaped bottom. The discharge orifice has a shutter in the form of a plunger valve with a plunger shaft descending vertically from an actuating mechanism above the hopper. A stirrer/scraper device with three arms moves concentrically about the plunger shaft, thereby stirring the powder in the holding container as well as scraping powder off the plunger and the funnel wall. Analogous to the foregoing examples, there is again the possible problem that the powder in the hopper, particularly in the funnel-shaped bottom part, may become compacted into a cohesive mass that simply rotates inside the hopper together with the stirrer/scraper device.
A solution to the aforementioned problems has previously been proposed in commonly-owned U.S. Pat. No. 7,284,574, which issued on 23 Oct. 2007. A device for metering powder quantities in the range from milligrams to a few grams with an accuracy of the order of one-tenth of a milligram has a holding container with a conically narrowing bottom portion and a discharge orifice with a shutter valve to regulate the powder flow. The device includes means for vibrating and/or tapping the container, and may further include a rotary stirring device which may in addition move back and forth in the direction of its axis. As a result, the powder will drop through the orifice under its own gravity without the use of an Archimedean feed screw.
Similar to the last-mentioned solution, a device proposed in commonly-owned and currently pending US published application 2004/0155069, has a cylindrical holding container with a cap containing an Archimedean feed screw oriented at a right angle to the cylinder axis of the container. When the metering device is in its working position in an automated powder-dispensing system, the holding container is on top and the cap with the feed screw at the bottom. Powder from the holding container enters the horizontal feed screw tunnel through an opening from above and is pushed along the tunnel to a discharge opening from where the powder drops into a receiving container. The flow rate of the powder discharged from the orifice is in this case controlled by varying the speed of the horizontal feed screw.
For powders that are not too cohesive and compactable, the last-mentioned powder-metering device with at least one of the means for vibrating, tapping, rotary stirring, and up/down motion of the stirrer shows the desired capability for metering powder quantities in the milligram-to-gram range with an accuracy of the order of one-tenth of a milligram or less. However, its performance still fell short in tests with corn starch which, as mentioned above, is often used as a test material to evaluate powder-metering devices for their capability to handle very compactable and cohesive powders.
In evaluating the state of the art of powder-metering devices, the applicant found that different designs are used for powders with different flow properties and/or for different dosage quantities and/or for different precision requirements. This leads to the further conclusion that there is an unmet need for a design concept where the same metering device has the ability to dispense any powder from free-flowing sand to a sticky and clogging material such as cornstarch, i.e. from grade 1 to grade 8 on the aforementioned rating scale for the stickiness of powders, in dosage quantities from under a milligram to 5 grams and over, and with a precision better than one-tenth of a milligram.
It is therefore the object of the present invention to provide a further improved powder-metering apparatus that is capable of dispensing measured doses of powder in the range from about 0.5 milligrams to 5 grams with an accuracy of the order of one-tenth of a milligram or less for powders that cover the full range from free-flowing (comparable to hourglass sand) to highly cohesive and clogging (for example cornstarch).